Embedded audio amplification systems such as those used in various types of portable communication devices such as smartphones, mobile phones, tablets etc. are often connected to external components such as headsets using 4-terminal 2.5 mm or 3.5 mm jack plugs on the headset. The first two terminals, often designated “tip” and “ring 1”, are standard stereo connections of left and right loudspeakers of the headset. The last two terminals are used for the microphone signal and the common headset ground. However, different vendors connect the microphone signal and the common ground to different terminals of the jack plug. Some vendors follow a standard of the Open Mobile Terminal Platform (OMTP), and others follow the opposite order. This situation is unfortunate because it is desirable if the embedded audio amplification systems are able to support headsets of both standards. Since the ground connection can be on different terminals or pins of the jack plug there is a need for a ground switch arrangement to support both standards and a detection mechanism which is capable of determining how the jack plug terminals are connected in any particular headset connected to the embedded audio amplification system.
US 2013/0142350 A1 discloses systems and methods for interfacing an audio system with a 4-pin headset jack, and more specifically to multi-standard headset with an integrated grounding switch. A jack sense system for detecting a jack configuration is provided and an arrangement of switches allows a system controller to connect the proper ground and microphone signal terminals of the jack plug to the corresponding terminals of the audio system.
The ground switch or switches required by such a ground switching mechanism will typically comprise a controllable semiconductor switch such as a MOSFET which can be switched between a conducting state and a non-conducting state by control of the gate voltage. However, the controllable semiconductor switch introduces a series resistance in the ground connection of the headset or earphone. Since the ground connection in the 4-pin jack plug is shared between the left and right loudspeakers and the microphone, any resistance therein introduces an undesirable audio signal component, i.e. a noise or error signal, on the microphone ground wire. This is particularly pronounced where the left and/or right loudspeakers of the headset exhibits relatively low impedance in the audio range, for example between 16 and 32 ohms. The low impedance of the loudspeaker(s) leads to a large flow of audio current in the common ground connection and through the ground switch in its conducting state or on-state.
Since the DC bias voltage for the microphone is fixed relative to the ground node or potential of the headset driver circuit, the noise or error signal represents distortion that may add an undesirable echo from the loudspeaker signal or signals to the output signal of the microphone preamplifier. The level of noise or error signal on the common ground connection may be lowered by decreasing a resistance of the ground switch in its on-state. However, this solution is often impractical for example when using semiconductor switches to perform the ground switching. In the latter situation, the larger dimensions required of the semiconductor switches lead to an increase of semiconductor die area and an accompanying increase of manufacturing costs of the semiconductor die. This increase of manufacturing costs is unacceptable for numerous types of cost-sensitive consumer equipment and applications.
The present invention addresses and solves this ground noise problem by using an error suppression circuit configured to sense the noise or error voltage at the terminal of the connector interface which is connected to the ground node or plane of the a headset driver circuit via the active/conducting ground switch. The error suppression circuit superimposes or adds a replica of the noise or error signal at the ground connected terminal to a DC bias voltage of the microphone as described in detail below.